The prior art carburetors, of the fixed venturi type, heretofore employed are required to provide a proper fuel-air mixture at idle engine operation as well as at maximum engine speeds with wide open throttle. The air flow at idle operation may be a little as 8.0 c.f.m. while at maximum engine speed and wide open throttle the air flow may be as great as 600.0 c.f.m. It can be seen that with such prior art carburetors it is attempted to provide proper fuel metering characteristics over a range of air flows varying in the order of 70:1. The main difficulty caused by the necessity to provide accurate metering control over a great airflow range arises from the fact that the carburetor which has a fixed venturi throat large enough to flow the required air capacity at conditions of wide open throttle, is also too large to generate a satisfactory metering signal or force at the venturi throat at low rates of air flow.
The general problem as set forth above was attempted to be solved by the prior art as by the development of, for example, four barrel carburetors having two or more staged barrels or bores so that only a select number of such bores were open to air flow during a first range of engine operating conditions and additional bores were opened to air flow during succeeding ranges of engine operating conditions. Such multistage prior art carburetors have not totally solved the problems and, indirectly, have often added new problems in such areas as, for example, achieving smooth transitional fuel flows as the carburetor is passing from a first stage to a second or third stage of its operation.
Others have heretofore attempted to obviate the necessity of a fixed venturi, or for that matter any venturi, by employing a variable positionable air valve within the induction passage upstream of a fuel metering valve operatively connected to such air valve. Spring means were employed to resiliently resist opening movement of the air valve which was urged in the opening direction by the air flow through the induction passage. Theoretically, the more the air valve opened the greater the rate of metered fuel flow to the induction passage. However, it can be readily appreciated that the metering accuracy of such prior art air valve carburetors was somewhat less than ideal especially when it is realized that springs usually have a spring rate tolerance of .+-. 8.0% while, under present emission requirements, fuel metering has a total tolerance of about 1.5%.
Heretofore, various forms of variable venturi carburetors have also been suggested primarily to provide for a greater range of required air flows. However, such carburetors have not been entirely successful in the past and are totally unacceptable in the present because of their inability to provide extremely closely controlled fuel metering requirements necessary to meet the various regulations relative to exhaust emissions. In the prior art, the variable venturi was often employed as both a variable venturi and a throttle valve with no additional throttle valve being employed downstream of such variable venturi. Consequently, the relatively very high engine developed manifold vacuum was applied to the various fuel discharge orifices resulting in an extremely high fuel metering depression. This, in turn, required that the fuel metering system, per se, had to be extremely sensitive to both air flow and change in value of the metering depression. Such sensitivity, as a practical matter, is impossible to attain especially in mass production where repeatability of performance over millions of units is essential.
Accordingly, the invention as herein disclosed and described is directed primarily to the solution of the above as well as other related problems.